An airpump or airlift pump is a pump which is powered by compressed air, being of simple construction, the only energy required is air. The air being used is usually compressed air which is done by a compressor or blower. The air is pumped into the lower part of a pipe that transports a liquid and usually bubbles into another larger pipe, and by buoyancy the air which has lower density than the liquid rises. By fluid pressure the liquid is taken in the ascendant air flow and moves in the same direction as the air. Calculation of the flow of liquid is possible using the two phase flow calculation used in physics. The drawbacks of airlift pumps are weak suction, low lift, frequent clogging, difficult flow control and unstable flow rate. These pumps are often used in situations like deep wells which are dirty and contain such abrasives as sand which could damage mechanical moving parts. In this application the compressor would be kept on the surface so that no mechanical parts are actually in the well. It is sometimes used in the process of waste water treatment where a small head is required; they can often found being used in ponds to aerate the water.